The present invention relates to heat exchangers, and is more particularly directed to heat exchangers which have tubes for transferring heat between a coolant liquid flowing through the tubes and a refrigerant fluid in contact with the exterior of the tubes. The present invention is more specifically directed towards tubes which have an internal rib enhancement and an external fin enhancement, and also towards an improved method for making such tubing.
In the condenser portion of certain refrigeration or air conditioning systems, a coolant fluid, such as water, is passed through heat transfer tubing while refrigerant vapor in contact with the exterior of the tubing changes state from vapor to liquid, giving up heat of condensation to the coolant liquid within the tubing. The external and internal configuration of the tubing is important in determining the overall heat transfer characteristics of the tubing, and hence in determining the efficiency of the system. With condenser tubing that has an internal rib enhancement and an external fin enhancement, the condensation activity takes place at the tips or extrema of the fin, and the condensate flows into the channels between the fins. The condensed liquid refrigerant fills the channels to a point at which the coolant drips out. An internal enhancement, in the form of spiral or helical ribs or fins, causes a swirling of the flowing coolant within the tube. This induces some turbulence, which breaks up laminar flow and thus also prevents any insulating barrier layer from forming at the inner wall of the tube.
Tubes that are given both an internal and external enhancement are described, for example, in the commonly-assigned U.S. Pat. No. 4,425,696. Although that patent is directed to an evaporator, rather than a condenser tube configuration, a heat transfer tube suitable for use as a condenser tube could be constructed on the same tube finning machine, omitting the step of rolling the fins that is described in that patent. Other finned tubes for heat transfer are described in U.S. Pat. Nos. 4,059,147 and 4,438,807.
In the tube finning machine employed in the production of this tubing, a cylindrical grooved mandrel within the tube produces the internal rib, while a tool gang of discs carried on a tool arbor produces a fin convolution on the exterior of the tubing. The force of the gang of discs on the metal tubing and against the mandrel causes the metal of the tubing to flow up between the discs to form the fins and down into the mandrel grooves to form the ribs. At the locations of the grooves, however, there is less force placed on the metal, and the tubing metal does not flow as far outward between the discs of the tool gang. As a result, there is a reduced height in the external fin at locations which correspond to crossings of the fins with the internal rib. This produces a visually noticeable Moire pattern in the fins. Generally, the external fin has a height of about 0.030 inches, but the extent of dip or shortening due to
this Moire imprint is about 0.005 to 0.008 inches.
As aforementioned, in a condenser tube the tips or extrema of the fin is where mos of the condensation activity takes place. However, because of the significant Moire reduction in height, where the fin crosses the path of the rib the amount of exposed fin is significantly reduced. The reduction in efficiency of condensation of refrigerant can exceed twenty-eight percent, as compared to a finned tube where the fin height is uniform over the circumference of the tube.
A way to produce condenser tubes with a uniform external fin height with an internal enhancement has long been sought, but no one has previously been able to produce such a tube.